It may be that the k2tog (especially if they were close to each edge) are contributing to the trapezoid shape. Since it's acrylic yarn, it won't help to block it but washing and drying according to the labed instructions may help. Keep a good count of the sts every row to make sure you're not picking up sts and if so, that you catch them early on. It will also help you figure out what you're doing to increase the extra sts. There's a lot of information here
about extra sts so you can see that you're not alone!